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Analysis of the interaction between plasmas and the graphite first wall in RFX-mod
Journal of Nuclear Materials 415 (2011) S274–S277
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Analysis of the interaction between plasmas and the graphite first wall in RFX-mod S. Barison b, A. Canton a, S. Dal Bello a, S. Fiameni b, P. Innocente a,⇑, A. Alfier a, S. Munaretto a, F. Rossetto a a b
Consorzio RFX, Associazione EURATOM/ENEA sulla Fusione, Corso Stati Uniti 4, 35127 Padova, Italy CNR-IENI, Corso Stati Uniti 4, 35127 Padova, Italy
a r t i c l e
i n f o
Article history: Available online 6 January 2011
a b s t r a c t In the RFX-mod Reversed Field Pinch experiment the wall is entirely covered by graphite tiles. The disadvantage of a graphite wall is that, except after wall conditioning procedures, such as boronization, lithization or glow discharge cleaning, the graphite becomes saturated with trapped Hydrogen, causing that during the discharge the particle recycling factor is approximately one and plasma density is entirely sustained by particle fluxes coming from the wall. In the paper the results of the analyses of graphite samples after their exposure to plasmas or wall conditioning treatments are presented. The topics addressed are: the deposition of Boron during different boronization treatments; the erosion and re-deposition of the Boron layer by Hydrogen plasmas; some remarks about the presence of Lithium in the wall since the beginning of lithization treatments. Ó 2011 EURATOM. Published by Elsevier B.V. All rights reserved.
1. Introduction RFX-mod is an experiment of magnetic confinement of thermonuclear plasmas in the Reversed Field Pinch magnetic configuration. The toroidal device has a major/minor radius of 2/0.46 m and it is designed to operate with Hydrogen plasmas, plasma currents up to 2 MA and discharges duration of about 0.5 s (flat top phase of about 0.3 s) [1]. The RFX-mod first wall is entirely covered by tiles made of polycrystalline graphite (manufacturer Carbone Lorraine, type 5680PT) in order to bear the strong thermal power deposition that can locally reach values of the order of tens MW/ m2 [2]. The disadvantage of a graphite first wall is that during the discharge the particle recycling factor is always close to unity and plasma density is entirely sustained by particle fluxes coming from the wall. In such a condition the density of the plasma is mainly determined by the status of the wall (i.e. the content of Hydrogen particles in the graphite) and by the characteristics of the plasma–wall interaction (PWI), whereas the typical methods to actively control density (like gas filling, puffing and pellet injection) proved to be ineffective on RFX-mod, as discussed in references [3,4]. In RFX-mod, H2 particles have been found to accumulate in the wall from shot to shot, causing a progressive increase of the working density. The only way to prevent this density rise is a frequent treatment of the graphite with cleaning procedures. On daily base, or even more frequently, whenever the operation density becomes
⇑ Corresponding and presenting author. Tel.: +39 0498295062; fax: +39 0498700718. E-mail address: [email protected] (P. Innocente).
too high, 20–60 min of RF assisted Glow Discharge Cleaning (GDC) [5] with flowing He gas are performed (two anodes at 180 toroidal degrees each other, inserted approximately in the center of the vessel, applied voltage up to 1000 V, current up to 1.2 A, He pressure 103 mbar). The treatment is effective in sputtering Hydrogen atoms trapped in graphite [5]: at RFX-mod we measured that 20–60 min typically remove 1–2 1021 H atoms and allow to restart operation at lower density, but they cannot avoid the progressive particle accumulation in the wall [4]. When the wall has been exposed to air or before high performance experimental campaigns, stronger treatments are applied (baking, GDC with H2 gas or with a mixture of He (90%) and B2H6 (10%) gas, hereafter called boronization) in order to get both a better control of density and a reduction of the impurity content of the plasma. For the same purpose, Lithium deposition on the wall by means of solid Lithium pellet injection in Helium plasmas (lithization) has been recently tested, whereas the first experiments of lithization by means of a Liquid Lithium Limiter are scheduled in August 2010 [6]. The critical role of the graphite wall on the control of density suggested the implementation of several diagnostic tools on RFX-mod, in order to get a better control of particle fuelling (shot by shot calibration of the piezo-electric valves used to fuel the discharge [3]), to monitor the PWI (cameras, thermal sensors [7,8]) and to track the accumulation of Hydrogen in the wall (by measuring shot by shot the number of particles supplied before and during the discharge and out-gassed after its end [3]). Very recently the diagnostic network has been extended with the installation of manipulators that allow the insertion of samples inside the vessel up to the first wall envelope surface. In this paper we present the results of the analyses of first graphite samples after their exposure to plasmas and wall conditioning treatments.
0022-3115/$ - see front matter Ó 2011 EURATOM. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.311
S. Barison et al. / Journal of Nuclear Materials 415 (2011) S274–S277
2. Experimental setup Since January 2010 two manipulators, in two different toroidal positions, have been available. One position (Pos. 1) is in correspondence of one of the two anodes for the GDCs, and samples are inserted from a port in the equatorial plane. The other position (Pos. 2) is vertical and 30 toroidal degrees far from the other anode (Fig. 1). At this position four samples are exposed at the same time, and can be replaced independently. A third manipulator will be available in the next months, at a toroidal position at 180° from the latter. So far the samples exposed to plasma have been cut from the same graphite blocks used to make the tiles of RFX-mod, in order to track the status of the wall of RFX-mod with samples as similar as possible to the tiles. The drawback is that graphite is a highly porous material that makes instrumental analysis quite difficult in particular when depth profiles are performed (see next paragraph). Other materials could be used for the samples (metals, like Molybdenum, or Silicon), as already done in the past on RFX [9], but their behavior is not easily extrapolated to the graphite wall. The exposed samples have been analyzed by field emission SEM (Scanning Electron Microscopy, SIGMA, Carl Zeiss) to highlight the morphology and to get some information on the surface elemental composition, and by SIMS (Secondary Ions Mass Spectrometry) to measure the depth profiles of the present species. In particular the SIMS profiles have been obtained eroding the sample by a monochromatic Oþ 2 primary ion beam (6 keV, 0.5 0.5 mm size) generated in a mass-filtered duoplasmatron ion gun (model DP50B, VG Fisons) and an EQS1000 (Hiden) mass energy analyzer with a sector field electrostatic energy analyzer and a quadrupole mass filter were used for negative and positive ion detection in counting mode. The erosion rate, measured by a Tencor P-10 mechanical profiler having a 2 nm vertical resolution, was
vertical sample POS.2
GDCanode anode GDC sections sections
equatorial sample POS.1 Fig. 1. Picture of RFX-mod vessel with the GD’s anode sections and sample insertion ports highlighted.
S275
estimated to be about 8 nm/min with the maximum available depth resolution (that however, in our samples, is limited by the porosity of the graphite as discussed later). The significant species followed by SIMS were: Carbon from the wall, Boron and Lithium coming from wall conditioning treatments and Oxygen as the main plasma impurity. Hydrogen and Helium coming from plasmas and GDCs are not directly detectable by SIMS. For the analysis, the signals of the different species have been normalized to the signal of Oxygen, to take into account the effect of Oxygen on all positive ions yield and hence to allow the comparison of the profiles of the same species on different samples.
3. Results and discussion SEM images of the samples highlight the roughness and porosity of the graphite used for the first wall of RFX-mod. Fig. 2, for example, is the image of a sample at two different scales. The picture shows a material made of several graphite grains and plates of various dimensions in the range 0.1–1 lm and deep pores which extend for fraction of microns in the thickness of the sample. The sample of the picture had been exposed to 100 s of RFX-mod plasma but SEM images of blank graphite show a very similar morphology. The samples analyzed by SIMS and described in this paper are summarized in Table 1. The topics addressed are: the deposition of Boron during two boronization treatments that differ for the duration, comparing nominal and experimental thicknesses and highlighting the toroidal non-uniformity in the deposition; the erosion and re-deposition of the Boron layer by Hydrogen plasmas; some remarks about the presence of Lithium in the wall since the beginning of lithization treatments. The effect of the high surface roughness and porosity of graphite on the depth resolution of the measurement has been evaluated by comparing different profiles taken on the same sample, and a 10% error on the depth measure was estimated. At a first glance the small error can surprise but it is probably due to the surface averaging performed from the beam size. Fig. 3 shows the profiles of Oxygen, Carbon, Boron and Lithium on the sample 2D_I, exposed at Pos. 1 (see Fig. 1) during a boronization that nominally deposited a 45 nm thick layer (on average) on RFX-mod wall. The computation of the nominal thickness takes into account the flux of Diborane continuously supplied during the GDC process and the percentage of Diborane, in terms of partial pressure, present in the vessel during the treatment, the latter being evaluated by mass spectroscopy RGA. All the signals show long tails that once again could be due to the presence of pores and high surface roughness and hence to different sampling depths, besides the effect of the in-depth SIMS resolution. Experimental Boron in-depth presence is about 25 nm (panel (b), solid line), about 40% lower than the nominal one. The Boron and Carbon
Fig. 2. SEM images (not at the same position) of a graphite sample exposed to 100 s of plasma.
Table 1 Samples description. Sample #
Toroidal location
Exposed to:
2D_I 2D_II 2D_V 9D_I 9D_II 9D_III
Pos. 1 (GDC’s anode 1)
45 nm thick boronization 25 s H2 plasma 160 nm thick boronization 45 nm thick boronization 45 nm thick boronization + 25 s H2 plasma 45 nm thick boronization + 25 s H2 plasma + 37 s He plasma with 38 Li pellets 160 nm thick boronization 160 nm thick boronization + 22 s H2 plasma 22 s H2 plasma 30 s He plasma with 52 Li pellets
Pos. 2 (30° from GDC’s anode 2)
9D_IX 9D_X 9D_XIII 9D_XIV
4500
Cps
3500
1400 1200
Position 1: Short Position 1: Long
1000 800 600 400
Short Long
200 0 1200
(a) Position 2: Short Position 2: Long
1000 800 600 400 200 0
(b) 0
50
100
150
200
250
300
350
400
Depth (nm)
Oxygen Blank Oxygen 2D_I
4000
Normialized cps
S. Barison et al. / Journal of Nuclear Materials 415 (2011) S274–S277
Normalized cps
S276
Fig. 4. Boron profiles of samples exposed at Pos. 1 (a) and Pos. 2 (b) to two different boronizations. Vertical lines correspond to nominal thicknesses of the Boron layers for the two treatments. y-axis as in Fig. 3.
3000 2500 2000
Normalized cps
1500 2.0 1000
(a) Boron 2D_I Carbon 2D_I
1.5 1.0 0.5
(b)
0.0 700 Lithium Blank Lithium 2D_I
Normalized cps
600 500 400 300 200
(c)
100 0 0
20
40 60 Depth (nm)
80
100
Fig. 3. Profiles of different species exposed at Pos. 1 to a boronization treatment. In the y-axis the raw signal in counts per seconds (cps) in the panel (a), the raw signal of the species normalized to the raw signal of Oxygen in the panels (b) and (c).
profiles, as expected, have reverse patterns (panel (b)), but the maximum of Boron is located at a depth of 15 nm and not at the surface. Further analyses are under way to address this point. Oxygen shows an increase in correspondence of the peak of Boron (panel (a)), suggesting a co-deposition of the two species during the boronization treatment, likely due to the presence of Oxygen as impurity inside the chamber. This picture is also confirmed by the fact that the analysis of the gas extracted from the chamber by the vacuum system shows a presence of Oxygen before the beginning of the boronization treatment and its reduction when voltage is applied to the anodes. The presence of Lithium in the surface of the sample (panel (c)) could be simply addressed as surface contamination but the comparison with the Lithium content of the blank sample, which has been handled in a similar way and which shows a much lower content of Lithium, could suggest a re-deposition on the sample of atoms that were present on the wall of RFX-mod since the previous campaigns of Lithium pellet injection. Fig. 4 compares Boron profiles measured at the two different toroidal positions and during two different boronizations. The
nominal thicknesses of the boronizated layers are 45 and 160 nm and are represented in the figure by two vertical lines. Panel (a) shows the Boron profiles at Pos. 1, the toroidal section of one anode, panel (b) those referred to the Pos. 2, the section at 30° from the other anode. At Pos. 1 the thickness of the layer for the longer boronization is well comparable to the nominal one, whereas at the other position the experimental thicknesses are shorter than nominal ones for both treatments. At both positions, the longer boronization gave not only a thicker deposit, but also a layer that seems richer in Boron, since the Boron signals are higher. The comparison between the panels shows that there is a strong toroidal non-uniformity in the deposition: only 30° far from the anode, the thickness of the layer drops to about one half, and the Boron content decreases. This non-uniformity will be verified in future boronization treatments by the mentioned above increased number of samples positions and inspecting toroidal uniformity of emitted light in conditioning discharge. To enlighten the redistribution of Boron during the Hydrogen plasma sessions, that was found to mitigate the non-uniformity, Fig. 5 compares at the two toroidal positions samples that were exposed to the short boronization (thin lines) and to about 25 s of H2 plasma following the short boronization (thick lines). The samples exposed to plasma at the two positions were not exactly in the same conditions, since that at Pos. 1 was a blank one, whereas the other had been previously exposed also to the boronization. After Hydrogen plasma, the Boron content in the sample at Pos. 1 gives the size of the re-deposition that can be compared with the variation of Boron profile at Pos. 2 (solid lines). The latter seems larger, but the presence of Boron on the sample at Pos. 2 could have facilitated the growth of the re-deposited Boron layer. Fig. 5 shows that Lithium also is redistributed throughout the wall (dashed lines), highlighting that the re-deposited layers are made of a mix of species. As already seen in the past [9], the re-deposited layer is also rich in Carbon, and this is in agreement with the fact that the influx of Carbon during Hydrogen plasmas is only halved with a boronized wall with respect to a not conditioned wall. The presence of Carbon in the layer could be minimized with a more homogeneous deposition of Boron on the whole wall during the boronization treatment, and this suggests that the wall conditioning plant of RFX-mod should be upgraded with the installation of other two anodes at least. So far, it is difficult to compare experimental Lithium profiles obtained from the samples to the Lithium layers nominally deposited on the wall during the lithizations, since nominal thicknesses
S. Barison et al. / Journal of Nuclear Materials 415 (2011) S274–S277
treatments. Fig. 6 shows the average of Lithium profiles in the first 30 nm of the samples. Data show a progressive increase of the redeposited Lithium as far as experimental campaigns with Lithium injection are performed, suggesting that Lithium is not easily removed from the graphite of RFX-mod. This could provide during vessel venting the production on the wall of lithium carbonate (Li2CO3) [10,11] that can strongly affect the achievement of good plasma discharges after restarting operations.
Normalized cps
500
Position 1 400
B: 25 s plasma Li: 25 s plasma B: Boronization Li: Boronization
300 200
(a)
100
S277
Normalized cps
0
Position 2
400
4. Conclusions
200
B: Boronization+25s plasma Li: Boronization+25s plasma B: Boronization Li: Boronization
100
(b)
300
0 0
20
40
60
80
100
Depth (nm) Fig. 5. Boron and Lithium profiles of samples exposed at Pos. 1 (a) and Pos. 2 (b) to boronization and following H2 plasma session. y-axis as in Fig. 3.
Lithization
Boronization
Lithization
600
Lithization
800
Boronization
Lithium (normalized cps)
1000
400
200
The analysis of the first samples exposed to RFX-mod plasmas and wall conditioning treatments has been done and in this paper the profiles of Carbon, Oxygen, Boron and Lithium collected at two different toroidal positions have been discussed. On samples exposed during the boronization of the wall we have found that the in-depth presence of Boron is better in agreement with the nominal estimations when long treatments are performed. Long boronizations seem to give also conditioned layers that are richer in Boron. On the other hand, the comparison of samples showed a strong non-uniformity of the Boron deposition, suggesting the need to upgrade the GD plant with the increase of the number of anodes. The evidence of erosion and re-deposition of the boronized layers have been found, as well as of the accumulation of Lithium in the tiles of RFX-mod since lithization campaigns are performed. Acknowledgments This work was supported by the European Communities under the contract of Association between EURATOM/ENEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References
0 9D_XIV
9D_XIII
9D_X
9D_IX
9D_III
9D_II
9D_I
Blank
Fig. 6. Average of Lithium profiles in the first 30 nm of several samples.
are of the order of 10 nm, comparable with the region of surface contamination and with the resolution of the measurement. Anyway it is interesting to analyze the Lithium content of (blank) samples sequentially exposed to plasmas and wall conditioning
[1] P. Sonato et al., Fusion Eng. Des. 161 (2003) 66. [2] P. Zaccaria et al., Fusion Eng. Des. 66–68 (2003) 289. [3] A. Canton et al., 35th EPS Conference on Plasma Phys. Hersonissos, 9–13 June 2008 ECA, vol. 32D, 2008, D-1.002. [4] A. Canton et al., Presented at 13th IEA/RFP Workshop, Stockholm, October 9– 11 2008. [5] J. Winter, J. Nucl. Mater. 161 (1989) 265. [6] A. Alfier et al., This Conference, P2-82. [7] P. Sonato et al., Fusion Eng. Des. 66–68 (2003) 161. [8] P. Zanca et al., J. Nucl. Mater. 363–365 (2007) 733. [9] L. Tramontin et al., J. Nucl. Mater. 266–269 (1999) 709. [10] M. Töwe, P. Reinke, P. Oelhafen, J. Nucl. Mater. 290–293 (2001) 153–157. [11] S.S. Harilal et al., Appl. Surf. Sci. 255 (2009) 8539–8543.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Analysis of the interaction between plasmas and the graphite first wall in RFX-mod S. Barison b, A. Canton a, S. Dal Bello a, S. Fiameni b, P. Innocente a,⇑, A. Alfier a, S. Munaretto a, F. Rossetto a a b
Consorzio RFX, Associazione EURATOM/ENEA sulla Fusione, Corso Stati Uniti 4, 35127 Padova, Italy CNR-IENI, Corso Stati Uniti 4, 35127 Padova, Italy
a r t i c l e
i n f o
Article history: Available online 6 January 2011
a b s t r a c t In the RFX-mod Reversed Field Pinch experiment the wall is entirely covered by graphite tiles. The disadvantage of a graphite wall is that, except after wall conditioning procedures, such as boronization, lithization or glow discharge cleaning, the graphite becomes saturated with trapped Hydrogen, causing that during the discharge the particle recycling factor is approximately one and plasma density is entirely sustained by particle fluxes coming from the wall. In the paper the results of the analyses of graphite samples after their exposure to plasmas or wall conditioning treatments are presented. The topics addressed are: the deposition of Boron during different boronization treatments; the erosion and re-deposition of the Boron layer by Hydrogen plasmas; some remarks about the presence of Lithium in the wall since the beginning of lithization treatments. Ó 2011 EURATOM. Published by Elsevier B.V. All rights reserved.
1. Introduction RFX-mod is an experiment of magnetic confinement of thermonuclear plasmas in the Reversed Field Pinch magnetic configuration. The toroidal device has a major/minor radius of 2/0.46 m and it is designed to operate with Hydrogen plasmas, plasma currents up to 2 MA and discharges duration of about 0.5 s (flat top phase of about 0.3 s) [1]. The RFX-mod first wall is entirely covered by tiles made of polycrystalline graphite (manufacturer Carbone Lorraine, type 5680PT) in order to bear the strong thermal power deposition that can locally reach values of the order of tens MW/ m2 [2]. The disadvantage of a graphite first wall is that during the discharge the particle recycling factor is always close to unity and plasma density is entirely sustained by particle fluxes coming from the wall. In such a condition the density of the plasma is mainly determined by the status of the wall (i.e. the content of Hydrogen particles in the graphite) and by the characteristics of the plasma–wall interaction (PWI), whereas the typical methods to actively control density (like gas filling, puffing and pellet injection) proved to be ineffective on RFX-mod, as discussed in references [3,4]. In RFX-mod, H2 particles have been found to accumulate in the wall from shot to shot, causing a progressive increase of the working density. The only way to prevent this density rise is a frequent treatment of the graphite with cleaning procedures. On daily base, or even more frequently, whenever the operation density becomes
⇑ Corresponding and presenting author. Tel.: +39 0498295062; fax: +39 0498700718. E-mail address: [email protected] (P. Innocente).
too high, 20–60 min of RF assisted Glow Discharge Cleaning (GDC) [5] with flowing He gas are performed (two anodes at 180 toroidal degrees each other, inserted approximately in the center of the vessel, applied voltage up to 1000 V, current up to 1.2 A, He pressure 103 mbar). The treatment is effective in sputtering Hydrogen atoms trapped in graphite [5]: at RFX-mod we measured that 20–60 min typically remove 1–2 1021 H atoms and allow to restart operation at lower density, but they cannot avoid the progressive particle accumulation in the wall [4]. When the wall has been exposed to air or before high performance experimental campaigns, stronger treatments are applied (baking, GDC with H2 gas or with a mixture of He (90%) and B2H6 (10%) gas, hereafter called boronization) in order to get both a better control of density and a reduction of the impurity content of the plasma. For the same purpose, Lithium deposition on the wall by means of solid Lithium pellet injection in Helium plasmas (lithization) has been recently tested, whereas the first experiments of lithization by means of a Liquid Lithium Limiter are scheduled in August 2010 [6]. The critical role of the graphite wall on the control of density suggested the implementation of several diagnostic tools on RFX-mod, in order to get a better control of particle fuelling (shot by shot calibration of the piezo-electric valves used to fuel the discharge [3]), to monitor the PWI (cameras, thermal sensors [7,8]) and to track the accumulation of Hydrogen in the wall (by measuring shot by shot the number of particles supplied before and during the discharge and out-gassed after its end [3]). Very recently the diagnostic network has been extended with the installation of manipulators that allow the insertion of samples inside the vessel up to the first wall envelope surface. In this paper we present the results of the analyses of first graphite samples after their exposure to plasmas and wall conditioning treatments.
0022-3115/$ - see front matter Ó 2011 EURATOM. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.311
S. Barison et al. / Journal of Nuclear Materials 415 (2011) S274–S277
2. Experimental setup Since January 2010 two manipulators, in two different toroidal positions, have been available. One position (Pos. 1) is in correspondence of one of the two anodes for the GDCs, and samples are inserted from a port in the equatorial plane. The other position (Pos. 2) is vertical and 30 toroidal degrees far from the other anode (Fig. 1). At this position four samples are exposed at the same time, and can be replaced independently. A third manipulator will be available in the next months, at a toroidal position at 180° from the latter. So far the samples exposed to plasma have been cut from the same graphite blocks used to make the tiles of RFX-mod, in order to track the status of the wall of RFX-mod with samples as similar as possible to the tiles. The drawback is that graphite is a highly porous material that makes instrumental analysis quite difficult in particular when depth profiles are performed (see next paragraph). Other materials could be used for the samples (metals, like Molybdenum, or Silicon), as already done in the past on RFX [9], but their behavior is not easily extrapolated to the graphite wall. The exposed samples have been analyzed by field emission SEM (Scanning Electron Microscopy, SIGMA, Carl Zeiss) to highlight the morphology and to get some information on the surface elemental composition, and by SIMS (Secondary Ions Mass Spectrometry) to measure the depth profiles of the present species. In particular the SIMS profiles have been obtained eroding the sample by a monochromatic Oþ 2 primary ion beam (6 keV, 0.5 0.5 mm size) generated in a mass-filtered duoplasmatron ion gun (model DP50B, VG Fisons) and an EQS1000 (Hiden) mass energy analyzer with a sector field electrostatic energy analyzer and a quadrupole mass filter were used for negative and positive ion detection in counting mode. The erosion rate, measured by a Tencor P-10 mechanical profiler having a 2 nm vertical resolution, was
vertical sample POS.2
GDCanode anode GDC sections sections
equatorial sample POS.1 Fig. 1. Picture of RFX-mod vessel with the GD’s anode sections and sample insertion ports highlighted.
S275
estimated to be about 8 nm/min with the maximum available depth resolution (that however, in our samples, is limited by the porosity of the graphite as discussed later). The significant species followed by SIMS were: Carbon from the wall, Boron and Lithium coming from wall conditioning treatments and Oxygen as the main plasma impurity. Hydrogen and Helium coming from plasmas and GDCs are not directly detectable by SIMS. For the analysis, the signals of the different species have been normalized to the signal of Oxygen, to take into account the effect of Oxygen on all positive ions yield and hence to allow the comparison of the profiles of the same species on different samples.
3. Results and discussion SEM images of the samples highlight the roughness and porosity of the graphite used for the first wall of RFX-mod. Fig. 2, for example, is the image of a sample at two different scales. The picture shows a material made of several graphite grains and plates of various dimensions in the range 0.1–1 lm and deep pores which extend for fraction of microns in the thickness of the sample. The sample of the picture had been exposed to 100 s of RFX-mod plasma but SEM images of blank graphite show a very similar morphology. The samples analyzed by SIMS and described in this paper are summarized in Table 1. The topics addressed are: the deposition of Boron during two boronization treatments that differ for the duration, comparing nominal and experimental thicknesses and highlighting the toroidal non-uniformity in the deposition; the erosion and re-deposition of the Boron layer by Hydrogen plasmas; some remarks about the presence of Lithium in the wall since the beginning of lithization treatments. The effect of the high surface roughness and porosity of graphite on the depth resolution of the measurement has been evaluated by comparing different profiles taken on the same sample, and a 10% error on the depth measure was estimated. At a first glance the small error can surprise but it is probably due to the surface averaging performed from the beam size. Fig. 3 shows the profiles of Oxygen, Carbon, Boron and Lithium on the sample 2D_I, exposed at Pos. 1 (see Fig. 1) during a boronization that nominally deposited a 45 nm thick layer (on average) on RFX-mod wall. The computation of the nominal thickness takes into account the flux of Diborane continuously supplied during the GDC process and the percentage of Diborane, in terms of partial pressure, present in the vessel during the treatment, the latter being evaluated by mass spectroscopy RGA. All the signals show long tails that once again could be due to the presence of pores and high surface roughness and hence to different sampling depths, besides the effect of the in-depth SIMS resolution. Experimental Boron in-depth presence is about 25 nm (panel (b), solid line), about 40% lower than the nominal one. The Boron and Carbon
Fig. 2. SEM images (not at the same position) of a graphite sample exposed to 100 s of plasma.
Table 1 Samples description. Sample #
Toroidal location
Exposed to:
2D_I 2D_II 2D_V 9D_I 9D_II 9D_III
Pos. 1 (GDC’s anode 1)
45 nm thick boronization 25 s H2 plasma 160 nm thick boronization 45 nm thick boronization 45 nm thick boronization + 25 s H2 plasma 45 nm thick boronization + 25 s H2 plasma + 37 s He plasma with 38 Li pellets 160 nm thick boronization 160 nm thick boronization + 22 s H2 plasma 22 s H2 plasma 30 s He plasma with 52 Li pellets
Pos. 2 (30° from GDC’s anode 2)
9D_IX 9D_X 9D_XIII 9D_XIV
4500
Cps
3500
1400 1200
Position 1: Short Position 1: Long
1000 800 600 400
Short Long
200 0 1200
(a) Position 2: Short Position 2: Long
1000 800 600 400 200 0
(b) 0
50
100
150
200
250
300
350
400
Depth (nm)
Oxygen Blank Oxygen 2D_I
4000
Normialized cps
S. Barison et al. / Journal of Nuclear Materials 415 (2011) S274–S277
Normalized cps
S276
Fig. 4. Boron profiles of samples exposed at Pos. 1 (a) and Pos. 2 (b) to two different boronizations. Vertical lines correspond to nominal thicknesses of the Boron layers for the two treatments. y-axis as in Fig. 3.
3000 2500 2000
Normalized cps
1500 2.0 1000
(a) Boron 2D_I Carbon 2D_I
1.5 1.0 0.5
(b)
0.0 700 Lithium Blank Lithium 2D_I
Normalized cps
600 500 400 300 200
(c)
100 0 0
20
40 60 Depth (nm)
80
100
Fig. 3. Profiles of different species exposed at Pos. 1 to a boronization treatment. In the y-axis the raw signal in counts per seconds (cps) in the panel (a), the raw signal of the species normalized to the raw signal of Oxygen in the panels (b) and (c).
profiles, as expected, have reverse patterns (panel (b)), but the maximum of Boron is located at a depth of 15 nm and not at the surface. Further analyses are under way to address this point. Oxygen shows an increase in correspondence of the peak of Boron (panel (a)), suggesting a co-deposition of the two species during the boronization treatment, likely due to the presence of Oxygen as impurity inside the chamber. This picture is also confirmed by the fact that the analysis of the gas extracted from the chamber by the vacuum system shows a presence of Oxygen before the beginning of the boronization treatment and its reduction when voltage is applied to the anodes. The presence of Lithium in the surface of the sample (panel (c)) could be simply addressed as surface contamination but the comparison with the Lithium content of the blank sample, which has been handled in a similar way and which shows a much lower content of Lithium, could suggest a re-deposition on the sample of atoms that were present on the wall of RFX-mod since the previous campaigns of Lithium pellet injection. Fig. 4 compares Boron profiles measured at the two different toroidal positions and during two different boronizations. The
nominal thicknesses of the boronizated layers are 45 and 160 nm and are represented in the figure by two vertical lines. Panel (a) shows the Boron profiles at Pos. 1, the toroidal section of one anode, panel (b) those referred to the Pos. 2, the section at 30° from the other anode. At Pos. 1 the thickness of the layer for the longer boronization is well comparable to the nominal one, whereas at the other position the experimental thicknesses are shorter than nominal ones for both treatments. At both positions, the longer boronization gave not only a thicker deposit, but also a layer that seems richer in Boron, since the Boron signals are higher. The comparison between the panels shows that there is a strong toroidal non-uniformity in the deposition: only 30° far from the anode, the thickness of the layer drops to about one half, and the Boron content decreases. This non-uniformity will be verified in future boronization treatments by the mentioned above increased number of samples positions and inspecting toroidal uniformity of emitted light in conditioning discharge. To enlighten the redistribution of Boron during the Hydrogen plasma sessions, that was found to mitigate the non-uniformity, Fig. 5 compares at the two toroidal positions samples that were exposed to the short boronization (thin lines) and to about 25 s of H2 plasma following the short boronization (thick lines). The samples exposed to plasma at the two positions were not exactly in the same conditions, since that at Pos. 1 was a blank one, whereas the other had been previously exposed also to the boronization. After Hydrogen plasma, the Boron content in the sample at Pos. 1 gives the size of the re-deposition that can be compared with the variation of Boron profile at Pos. 2 (solid lines). The latter seems larger, but the presence of Boron on the sample at Pos. 2 could have facilitated the growth of the re-deposited Boron layer. Fig. 5 shows that Lithium also is redistributed throughout the wall (dashed lines), highlighting that the re-deposited layers are made of a mix of species. As already seen in the past [9], the re-deposited layer is also rich in Carbon, and this is in agreement with the fact that the influx of Carbon during Hydrogen plasmas is only halved with a boronized wall with respect to a not conditioned wall. The presence of Carbon in the layer could be minimized with a more homogeneous deposition of Boron on the whole wall during the boronization treatment, and this suggests that the wall conditioning plant of RFX-mod should be upgraded with the installation of other two anodes at least. So far, it is difficult to compare experimental Lithium profiles obtained from the samples to the Lithium layers nominally deposited on the wall during the lithizations, since nominal thicknesses
S. Barison et al. / Journal of Nuclear Materials 415 (2011) S274–S277
treatments. Fig. 6 shows the average of Lithium profiles in the first 30 nm of the samples. Data show a progressive increase of the redeposited Lithium as far as experimental campaigns with Lithium injection are performed, suggesting that Lithium is not easily removed from the graphite of RFX-mod. This could provide during vessel venting the production on the wall of lithium carbonate (Li2CO3) [10,11] that can strongly affect the achievement of good plasma discharges after restarting operations.
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The analysis of the first samples exposed to RFX-mod plasmas and wall conditioning treatments has been done and in this paper the profiles of Carbon, Oxygen, Boron and Lithium collected at two different toroidal positions have been discussed. On samples exposed during the boronization of the wall we have found that the in-depth presence of Boron is better in agreement with the nominal estimations when long treatments are performed. Long boronizations seem to give also conditioned layers that are richer in Boron. On the other hand, the comparison of samples showed a strong non-uniformity of the Boron deposition, suggesting the need to upgrade the GD plant with the increase of the number of anodes. The evidence of erosion and re-deposition of the boronized layers have been found, as well as of the accumulation of Lithium in the tiles of RFX-mod since lithization campaigns are performed. Acknowledgments This work was supported by the European Communities under the contract of Association between EURATOM/ENEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References
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Fig. 6. Average of Lithium profiles in the first 30 nm of several samples.
are of the order of 10 nm, comparable with the region of surface contamination and with the resolution of the measurement. Anyway it is interesting to analyze the Lithium content of (blank) samples sequentially exposed to plasmas and wall conditioning
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